The purpose of this cross-sectional study is to determine the prevalence, correlates and sequelae of club drug use among an economically disadvantaged, racial/ethnically diverse population of substance users in New York City (NYC) in order to provide information on the associations between use of club drugs and high risk sexual behavior in this population. This information can be instrumental to comprehensive substance abuse and HIV prevention efforts in economically disadvantaged communities. Club drug use [a collective term for ecstasy, GHB, ketamine, LSD, methamphetamines, PCP, and flunitrazepam] is increasing in NYC based on data from indicator surveillance, the medical examiner, and hospital reports. In our ongoing surveys of injection drug users in Harlem, a higher number of lifetime sex partners was seen among the 30% with lifetime use of LSD and PCP. While sexual risks with heroin and cocaine in their various forms has been characterized in minority inner city communities, the degree to which club drugs, alone or in combination with other illicit drugs, are independently associated with risky sexual behavior and infections remains an open question. In this study we will enroll 2000 substance users using systematic recruitment techniques in eight disadvantaged neighborhoods within four boroughs of NYC. Eligible and consenting participants will undergo an interview and specimen collection. The study will address three aims: (1) to estimate the prevalence of club drugs (separately and combined) among persons who report injection and non-injection drug use, and to report personal characteristics (e.g. demographics, other illicit drug use), place (e.g., by neighborhood) and time (e.g., time trends within a multi-year sampling scheme) factors associated with club drug use; (2) to identify and compare the predisposing factors (e.g., availability, opportunity, family and peer influences, neighborhood characteristics) and specific circumstances of initiation and subsequent purchases of club drugs and to compare these factors for club drug use separately whether or not participants use inhaled or injectable forms of heroin, cocaine alone or together; (3) to identify and compare sexual risk behaviors among club drug users and non-club drug users in a population defined by active heroin, crack or cocaine use, after accounting for other covariates; and (4) to identify and compare the HIV and HCV seroprevalence among club drug users and non-club drug users.